1. Field of the Invention
The present invention relates to a control method of a jitter buffer that temporarily stores communication packets in the VOIP (Voice Over Internet Protocol).
2. Description of the Related Art
In recent years, the internet telephone has remarkably been spreading, and the sound quality thereof has been improved to such a level that practical inconveniences can hardly be found. And, the means to attain this level has adopted a method, which temporarily stores received packets in a jitter buffer, and thereby absorbs jitters accompanied with the packets. The extent of jitters in the packet transmission through the internet is about 100 ms, however the frame length in the packet transmission conforming to the International Standard G.729 is 10 ms. Accordingly, the jitter buffer needs a capacity to contain the packets of 10 frames at the minimum. However in the VOIP, if the capacity of the jitter buffer is too large, it will produce a delay time proportional to the capacity of the jitter buffer, after the counterpart finishes talking. This delay will interrupt the conversation, and further make produced echoes stand out, which deteriorates the sound quality of talks.
In order to solve this problem, a conventional technique, for example, sets a “disposal starting threshold” and a “disposal ending threshold” in the jitter buffer, executes the disposal processing of the packets according to the contents of voice data to thereby restrict the capacity of the jitter buffer, and inserts the “voice data of minute noises” to prevent interrupting voices and sounds (patent document 1: the Japanese Published Unexamined Patent Application No. 274829/2001).
Or, another conventional technique measures an available residual quantity of a reception buffer (jitter buffer), determines a clock frequency for setting the read timing in order that the available residual quantity always be within a predetermined range, and thereby restricts the capacity of the reception buffer (patent document 2: the Japanese Published Unexamined Patent Application No. 261613/1997).
However, the technique disclosed in the patent document 1 determines to or not to apply the disposal processing according to the contents of the voice data; accordingly, if there occurs a burst of jitters, it will lead to extinguishing a large quantity of packets, which produces distortions on reproduced voices. And, if such a processing is executed to a silent interval, such distortions will not be produced; however a calculation processing of the sound pressure becomes necessary, and the load to the software and hardware becomes increased accordingly.
Or, the technique disclosed in the patent document 2 determines the clock frequency for setting the reading speed by measuring the available residual quantity of the reception buffer, and controls the capacity of the reception buffer. However, the document does not disclose as to what kind of countermeasures can be taken, when there occurs a burst of jitters and the available residual quantity is nonexistent.